Dielectric insulation media in liquid or gaseous state are conventionally applied for the insulation of an electrical active part in a wide variety of electrical apparatuses, such as switchgears or transformers.
In medium or high voltage metal-encapsulated switchgears, for example, the electrically active part is arranged in a gas-tight housing, which defines an insulating space, said insulation space comprising an insulation gas usually with a pressure of up to several bars and separating the housing from the electrically active part, thus preventing flow of electrical current between housing and active parts. Metal-encapsulated switchgears allow for a much more space-saving construction than switchgears which are mounted outdoors and are insulated by ambient air. For interrupting the current in a high voltage switchgear, the insulating gas further functions as an arc extinction gas.
Conventional insulation gases with high insulation and switching performance have some environmental impact when released into the atmosphere. So far, the high global warming potential (GWP) of these insulation gases has been coped with by strict gas leakage control in gas-insulated apparatuses and by very careful gas handling.
Conventional environment-friendly insulation gases, such as dry air or CO2, have quite a low insulation performance, thus requiring a very unfavourable increase in gas pressure and/or insulation distances.
For the reasons mentioned above, efforts have been made in the past to replace the conventional insulation gases by suitable substitutes.
For example, WO 2008/073790 discloses a dielectric gaseous compound which—among other characteristics—has a low boiling point in the range between −20° C. to −273° C., is preferably non-ozone depleting and which has a GWP of less than about 22,200 on a 100 year time scale. Specifically, WO 2008/073790 discloses a number of different compounds which do not fall within a generic chemical definition.
Further, U.S. Pat. No. 4,175,048 relates to a gaseous insulator comprising a compound selected from the group of perfluorocyclohexene and hexafluoroazomethane, and EP-A-0670294 discloses the use of perfluoropropane as a dielectric gas.
EP-A-1933432 refers to trifluoroiodomethane (CF3I) and its use as an insulating gas in a gas-insulated switchgear. In this regard, the document mentions both the dielectric strength and the interrupting performance to be important requirements for an insulating gas. CF3I has according to EP-A-1933432 a GWP of 5 and is thus considered to cause relatively low environmental impact. However, because of its relatively high boiling point of −22° C., CF3I is taught to be mixed with CO2. The proposed gas mixtures have only around 80% of the specific insulation performance of a pure conventional insulation medium. This has to be compensated by an increased gas pressure and/or by larger insulation distances.
In the search for a suitable substitute, it has been found that by using fluoroketones having from 4 to 12 carbon atoms, an insulation medium can be obtained which has high insulation capabilities, in particular a high dielectric strength, and at the same time an extremely low global warming potential. This invention has previously been filed as international patent application No. PCT/EP2009/057294.
German Utility Model DE 20 2009 009 305 U1 and German Patent DE 10 2009 025 204 B3 also relate to a switching device having an encapsulation that is filled with a filling medium comprising a fluoroketone.
Despite of the good dielectric strength of the fluoroketones according to international patent application No. PCT/EP2009/057294, the insulation performance of the respective insulation medium comprising the fluoroketone is often limited due to the relatively high boiling points of the fluoroketones.
This is particularly the case for applications in a low temperature environment. In this case, only a relatively low saturated vapour pressure of the fluoroketone can be maintained without fluoroketone becoming liquefied. This limits the achievable fluoroketone molar ratio in the gaseous phase and would make necessary an increased filling pressure with conventional insulating gases.
For example, the minimal permissible operating temperature of high or medium voltage gas-insulated switchgear (HV-GIS or MV-GIS) can be typically −5° C. At this temperature, for obtaining a dielectric performance comparable to conventional high-performance insulation media, the required filling pressure of an insulation medium comprising e.g. a fluoroketone having 6 carbon atoms, e.g. C2F5C(O)CF(CF3)2 or dodecafluoro-2-methylpentan-3-one, may still be relatively high and could exceed the filling pressure that can be withstood by usual housing constructions, which is typically about 7 bar for HV GIS applications.
Alternatively or additionally to increasing the filling pressure, the system can be heated (as shown in our PCT/EP2009/057294). If using for example a pure fluoroketone having 6 carbon atoms, e.g. C2F5C(O)CF(CF3)2 or dodecafluoro-2-methylpentan-3-one, as the insulation medium, heating to more than 50° C. would be required to achieve a sufficient saturated vapour pressure of the fluoroketone and to obtain the desired insulation performance for more demanding high voltage applications. Such heating is not always feasible or recommended both for economic and ecologic and reliability reasons.